Technical Field
The present invention relates to a process for producing nitrogen by cryogenic distillation of atmospheric air.
Several processes are known in the art for the production of nitrogen by cryogenic distillation of atmospheric air. Past process schemes can be classified into three major categories based on the method used to provide the required refrigeration for the process: oxygen-rich gas expansion, air expansion and nitrogen-rich gas expansion.
A very well-known process for the production of nitrogen is the single-column process with oxygen-rich or waste expansion. In this process, the dried and cleaned compressed air is cooled to near its dew point and fed to the bottom of a distillation column to obtain a fraction rich in nitrogen at the too and a liquid fraction rich in oxygen at the bottom. The bottom fraction is than expanded to lower pressure and is vaporised in the overhead condenser of the column against the condensing nitrogen-rich stream at the top of the column. Part of the nitrogen-rich stream can be recovered as nitrogen product. The condensed nitrogen-rich stream is returned to the top of the column as reflux liquid. The vaporized oxygen-rich stream is expanded isentropically in an expander to provide necessary refrigeration for the process. A major disadvantage of this process is the poor recovery rate of nitrogen. This is due to the fact that the recovery is limited by the equilibrium between the feed air and the oxygen-rich liquid at the bottom of the distillation column. Several techniques are known for improving the performance of this single-column process by adding a reboiler at the bottom of the distillation column. This reboiler will displace the phase equilibrium allowing higher product recovery rate. In order to provide the reboil for the bottom reboiler, an oxygen-rich stream can be recycled, compressed and condensed in the reboiler. However, this solution requires the recirculation and compression of an oxygen-rich stream and would be expensive since special material of construction and special precautions must be utilized to avoid the hazard associated with the handling of an oxygen-rich stream.
A reboiler using condensing air may be used as described, for example, in European patent application 183,446 and U.S. Pat. No. 4,617,037. This solution has a relatively narrow range of nitrogen pressure (about 3 to 3.5 bar absolute). Product compression is required for pressure above 3.5 bar, otherwise sharp reduction in process efficiency will occur.
A reboiler using a nitrogen cycle can be quite advantageous by contrast. For example, the recycle compressor and the product compressor can be combined as a single compressor to yield a more cost-effective plant. U.S. Pat. No. 4,665,918 describes an air separation process where two nitrogen reboilers are used in a single distillation column process. A bottom reboiler condenses high pressure recycled nitrogen to provide a first reboil fraction. An intermediate reboiler condenses lower pressure nitrogen to provide additional reboil. This process requires complex equipment and multi-stage recycle compression machinery. Another inconvenience of this process is the relatively low pressure of the feed air: for the process to be efficient, the waste stream pressure leaving the process must be kept low at slightly above atmospheric pressure. This translates into correspondingly low column pressure which in turn results in low feed air pressure. The water and carbon dioxide removal which is usually necessary in the front-end clean-up equipment becomes more costly and more energy intensive since more heat must be applied to regenerate the purification equipment. Furthermore, the arrangement of the expansion turbine discharging into an intermediate reboiler tends to limit the flexibility of this process when it comes to varying the cold production of the process: a change in expander flow would affect the column operation and product purity.
U.S. Pat. No. 4,400,188 discloses the use of a single-column process with bottom reboiler and oxygen-rich gas expansion. Nitrogen recycle is used to provide additional reboil and reflux. The major drawback of this process is the hazard associated with the expansion of oxygen-rich gas in the turbine machinery. Special materials and precautions must be utilized to minimize the risk. Furthermore, this process requires the distillation column to operate at relatively high pressure, which in turn translates into higher recycle flow rate for a given product recovery rate. This will result in higher power consumption.
Therefore, a need exists for a process which simultaneously enables a good nitrogen extraction yield, while keeping the apparatus cold by expansion of an oxygen-poor gas in a turbine.
These objects and others which will become more apparent in view of the following, are provided by a process for producing gaseous nitrogen from a mixture to be treated in a distillation column, said mixture comprising nitrogen and oxygen, said process comprising the steps of:
a) compressing the mixture to be treated to a pressure at least equal to the column pressure;
b) cooling the compressed gas mixture and subjecting at least a first portion of said cooled mixture to expansion through a turbine to produce the required refrigeration and thereafter fractionated distillation in the column to obtain, at the bottom of the column, an oxygen-enriched fraction and, at the top of the column, a nitrogen-enriched fraction;
c) drawing off at least a portion of the nitrogen-enriched fraction as nitrogen product;
d) compressing the remaining portion of the nitrogen-enriched fraction;
e) recycling at least a portion of the compressed remaining portion of the nitrogen-enriched fraction to the bottom reboiler of the column where it condenses to provide reboil for the column;
f) introducing at least a portion of the condensed nitrogen stream of step e) to the top of column as additional reflux; and
g) drawing off an oxygen-enriched fraction in liquid form from the bottom of the column and expanding at least a portion of said fraction to a pressure less than the column pressure and vaporizing this portion by heat exchange with the condensing nitrogen-enriched fraction at the top of the column.
According to another embodiment, the invention also relates to a process for producing gaseous nitrogen from a mixture to be treated in a distillation column, said mixture comprising nitrogen and oxygen, said process comprising the steps of:
a) compressing the mixture to be treated to a pressure at least equal to the column pressure;
b) further compressing a first portion of said compressed gas mixture into a compressor driven by a turbine, cooling said further compressed gas mixture and expanding it through said turbine and thereafter fractionating by distillation said expanded gas mixture in the column to obtain, at the bottom of the column, an oxygen-enriched fraction and, at the top the column, a nitrogen-enriched fraction;
c) drawing off at least a portion of the nitrogen-enriched fraction as nitrogen product;
d) compressing the remaining portion of the nitrogen-enriched fraction;
e) recycling at least a portion of the compressed remaining portion of the nitrogen-enriched fraction to the bottom reboiler of the column where it condenses to provide reboil for the column;
f) introducing at least a portion of the condensed nitrogen stream of step e) to the top of column as additional reflux; and
g) drawing off an oxygen-enriched fraction in liquid form from the bottom of the column and expanding at least a portion of said fraction to a pressure less than the column pressure and vaporizing this portion by heat exchange with the condensing nitrogen-enriched fraction at the top of the column.
According to other embodiments of the invention, the expansion turbine drives a compressor to additionally compress the first portion of the compressed gas mixture, before cooling it and expanding it in the turbine.
In another embodiment of the invention, an additional higher pressure column is provided with the distillation column to remove from compressed air the light products such as H.sub.2, He and Ne from the nitrogen product to make very pure nitrogen product.
According to a further embodiment of the invention, compressed air is expanded in the turbine only for refrigeration of the column, the expanded air being e.g. vented with the waste stream from the column after heat exchange in the main heat exchanger. One might use either a single turbine, or a compressor driven by said turbine.
It is also possible in another embodiment, according to the process of the invention, to withdraw a portion of the nitrogen-rich recycling gas and expand it in a turbine to make refrigeration and recombine it with the nitrogen-rich stream and simultaneously use an air turbine to expand a portion of the feed cooled gas mixture (air) for refrigeration and for feeding the distillation column.
In all of the above embodiments, it is usually preferred to have the pressure of the compressed gas mixture (air) greater than the column pressure where distillation is made. In that case, The remaining portion, if any, of said cooled mixture of step b) is usually expanded through a valve and introduced into the column as an additional feed. This introduction is preferably done at an intermediate stage which is usually above the point of introduction of said first portion of said cooled mixture in the column. According to the invention, the nitrogen product is either withdrawn from the column as a gas or as a liquid. Both can be withdrawn when there is a need for both liquid and gaseous product.
Also, according to one further embodiment of the invention, the process comprises a liquid assist step (nitrogen, oxygen-rich or both) to provide e.g. more refrigeration during high demand period. The liquid (nitrogen or oxygen-rich) necessary for this liquid assist is stored from the column during periods of lower demand.